Smelting process of recovering chemicals from the black liquor derived in pulp mills



Jan. 24, 1950 E. A. J. GAGLIARDI ET AL SMELTING PROCESS OF RECOVERING CHEMICALS FROM THE BLACK LIQUOR DERIVED IN PULP MILLS Filed Aug. 8, 1946 IN V EN TOR S Patented Jan. 24, 1950 UNITED STATES PATENT OFFICE SMELTING PROCESS? OF RECOVERING CHEMICALS FROM THE BLACK LIQUOR DERIVED IN PULP MILLS.

Emilio A. J. Gagliardi and Charles Mellor, Buenos Aires, Argentina, and Alexander L. Hamrn, New York, N. Y.; said Hammassignor to Combustion- Engineering-Superheater' Inc.-

ApplicationAugust 8, 1946, Serial No. 689,154

3' Claims. 1

This invention relates to chemical recovery systems for thesulphate process of pulp manufacture for papermills and the like; and its nature. objects and advantageswill be best understood from the following:

In the sulphate process employed in pulp manufacture, the mixture of sodium hydrate or caustic soda (mom and sodium sulphide active alkali in theproportion thatitlsdlsed up. The resulting action of'this reserve soda on the wood is assumed to be less drastic and toaccount forthe improved'fiber-"and yield; Thus itis generally known that the amount of sodium sulphide present in the white liquor for'the digester, controls theresults produced by the sulphate process. If too small aqua'ntit' of sodium sulphide is present, the"cook or pulp produced, yields a shorter broomier fiber, resulting in a weaker fiber; While it'is'not definitely known just what are the reactions'w-hich control the type of fiber produced, itxis nevertheless known. that the presence of sodiumsulphide performsa' definite function in stopping or checking. the

drastic action. of sodium, hydrate. when active:

alone, as inthe soda process;

The term sulphidity as used by the" paper manufacturer, is. the ratio Ofa sodium sulphide- (Nazs) to the total alkali present in the'liquors. This sulphidity ratio ist'a-ken aiter'the green liquors from the recovery furnace are treated with lime and becomewhatarethen known. as white liquors, whichare the liquorsnow ready for reuse in the digesters. The. total alkaliis the sum. of. all. the sodas mfiesentin, the; white liquors. Tobe. able to the sulphidity-ratio at. will. andv with ease. and. yet: ensure. satisfactory operation-of. the, recovery 'iurnacevis. of great.

importance to theprocess.

In the process,. wood chips are cooked inthe. white liquor. within thed-igesters and the resulting pulp is washed and prepared for; shipment" or use. The removed liquonknown. as black liquon. is. passed. through. e-yaporators. to. obtain.- greater concentration, and, is. then. burned. in

special furnaces: to removeorganic matter, thedissolved non-fibrous portion of the Wood; Be-

fore entering the. furnacev sodium sulphate; (NaaSOr), a drycrystalline substance known as. The. black liquor is a;complex mixture of organic and inorganic compounds of carbon sulphur and 5'- salt/cake, is a'ddedto the black liquor.

sodium; in many forms. When heatv is. applied,

sodium carbonate '(NasCoz), sodium sulphide. (NazS), traces of Caustic. soda (NaOH), small.

amounts of sodium sulphate (NazSOO, and other sodium-sulphurcompounds appear in the smelt. Duringtheburning'and recovery process within the furnace, the sodium sulphate in the liquors,

"" reacts with the carbon present and is. reduced to sodium sulphide, which, as explained above, has a definite and-'important'place in the process. The smelt from the furnace is dissolved in. a

solution of weak liquor, and the mixture becomes known as "green liquor." Green liquor contains: sodium carbonate, atrace of caustic soda, sodium sulphide, and traces of other salts. Slaked lime is added to the green liquor and combines with the-sodium carbonate to form caustic soda and calcium carbonate, which last is filtered out.

The resulting fluid is the white liquor referred to above, containing 'sodium-hydrate-and sodium sulphide, and it is' then ready for reuse as a solvent in the digester process.

The sulphidity of the white liquor is varied" byvarying-the amountof salt'cake added to the black liquor prior to its entry intothe furnace. This added charge'changes the general performance of the so-called ashfonthe hearth andwill be referred toagain;

The concentrated black liquor, after having been heated to a predetermined temperature, is sprayed under pressure into space in the furnace, where-it is dried-and partly volatilized by the heat from the burn-ing. combustible I constituents; in

the liquor. Part of the combustion occurs in the space, but the larger part takes place on the.-

hearth, at the bottom-of theriurnace, onto which the sufiiciently dried portions of thesprayed liquor. fallsas so-called ash.

The. combustion inthe furnace occurs prefer-- ably in the. presence .of heat. absorbing surfaces. such as water. cooled. tubes, forming part of a The,- recovery. units are self-- steamv generator. so' 'sustaining, thatis, there. is. a. commensurate amount of heatrequi-red tobe. carried into the furnace by the black liquor to compensateeior heat given. up to the .absorhingsurf aces and the mg ofthecombustiblercunstituents on the hearth;

heat required'ior. the .successfuhdrying and; burn.--

- esses and purposes.

Thus a satisfactory black ash the reduction of NazSOr are obtained.

In the past, the heat value of the black liquor amounted to about 7000 B. t. u.s per 1b., which was adequate for successful, self-sustaining operation. Recently, however, paper mills using the sulphate process have found it highly desirable to remove what is termed the soap. This soap is a light substance consisting of the fats, resins and turpentine of the wood, and has a high calorific value, on the order of about 14,000 B. t. u.s per lb. In this recent practice, about of the dry solids of the liquor is removed in the form of soap, which is sold for other proc- Through this practice the heating value per lb. of dry solids of the black liquor is robbed of about 300 B. t. u.s to 400 B. t. u.s per lb.

Moreover the sulphate process has been recently adapted or coupled to bleaching processes in which a larger amount of chemical is necessary to obtain the proper fiber for the bleaching processes. Through the combination of soap removal and higher chemical charge to the furnace, the black liquors produced in these recent practices have a heating value of only from about 6000 to 6300 B. t. u.s per 1b., instead of the earlier heat values of around 7000 B. t. u.s per lb. This reduction of heat value seriously interferes with satisfactory and self-sustaining furnace operation and substantially impairs recovery.

Thus it has been found in smelting the lower heat value liquors above referred to, that there is a tendency for the liquors, when they are damp, to ball up or stick together in the furnace, making it diflicult for the air to penetrate to the combustibles therein, which is so necessary to produce the steady, free-burning and free smelting properties of the black ash bed on the hearth. When this condition occurs, what the operators refer to as a black or wet hearth, results. When the black ash bed thus wets down, it looses ignition.

One of the primary objects of the invention is to overcome the above difficulties which have arisen in modern operation. throu h the maintaining of the desired ashing conditions on the hearth, to secure efilcient recovery of chemical, while at the same time securing control of sulphidity.

How the foregoing. together with such other objects and advantages as may hereinafter appear. or are incident to our invention. are realized is illustrated in the accom an ing drawing which is a more or less dia rammatic side elevation of a recovery unit, partly in section, embodying the invention.

The recovery furnace is preferably of the time shown in the United States patent to Hamm, No. 2.319.399. In brief, the furnace is tall and is comprised of a furnace section proper A. opening into a chamber B located immediately thereabove.

At the bottom of the furnace A there is diagrammatically shown a refractory hearth D, the dimensions of which may vary. Furnace A and chamber B are lined with bare Water-walled tubes connected into the circulation of the boiler C. By far the greatest amount of steam is developed in the wall tubes. For further details of, the furnace boiler unit and its operational characteristics, reference may be had to the aforesaid Hamm patent.

Air is introduced into the lower part of furash which accumulates on the hearth, the amount of air being suflicient for reducing purposes. The ducts I30 deliver into the space above the ash bed and supply the balance of the air needed for combustion.

The black liquor at the desired density (say from about 60% to about 70% solids), at a pressure of say from about 10 lbs. to about 25 lbs. p. s. i. and at a temperature of say from about F. to about 240 F., preferably around 235 F., is introduced into the furnace through the nozzles I3. To this end a variable speed controllable pump I0, pumps the concentrated black liquor through one or more heaters II and pipe I2 to the burners.

In its flight through and descent in space in the furnace, the black liquor particles are dried and partly volatilized by the heat from the burning combustible constituents in the liquor, the combustion occurring as above pointed out, partly in space and partly on the hearth.

The manner in which the operational difliculties, incident to modern low heat value, high chemical content, black liquors, are overcome while at the same time control of sulphidity is obtained, will now be described. Broadly speaking, we add, at a suitable point or points, a material or materials which, while enriching the black liquor in a practical manner to restore the original heat value thereof and to prevent the tendency of the black liquor to agglomerate on the hearth and form a wet bed and to permit of a more uniform penetration of the air through the bed, also has to do with the control of sulphidity. The material which we prefer to use for performing these functions is low ash coke, either in ground or in pulverized form. The sulphur contained in the coke will add beneficial sulphur to the system. Coals, petroleum cokes, or other similar fuels high in carbon and low in ash and which contain sulphur, may be used, in a similar form or forms of physical condition.

The black liquor, after having been concentrated to a desired density in evaporators (not shown), is introduced into the cascade evaporator I from which it is delivered into a mixing tank 8 and thence via pipe 9 to pump I0 and ultimately to the nozzles I3. In the mixing tank 8, the black liquor is mixed with salt cake delivered thereto from the surge bin I4 by the feeder I5 and pipe IS. The feeder I 5 is operated by a variable speed controllable motor. The mixing tank 8 is provided with stirring devices such as paddles.

According to the preferred form of the invention, the coke is also delivered to the mixing tank 8 whereby the contents of salt cake, coke and black liquor are thoroughly mixed. To this end coke received at the plant is stored in a storage bin '22 from whence it is delivered to the surge bin I! by the conveyor 23, after having been'ground to proper size in the mill 24. From the surge bin II, the coke is delivered to the mixing tank 8 by means of the feeder I8 and pipe I9. The feeder I8 is driven by a variable speed controllable motor.

The salt cakeis stored in a bin 20 and is led to the surge tank I4 by means of the conveyor 2|.

In operation, when the mixed substances are sprayed into the furnace, the particles of coke and salt cake are dispersed throughout the dry solids of the black liquor after dehydration has The presence of the coke particles taken place. dispersed throughout the dry solids on the hearth is advantageous. stantially dry and porous, have capacity to separate the sticky particles of black liquor and pre- The coke particles, while subgamma;

vent the balling up or. binding. ;together of the black ash collecting on the hearth. The separated and dispersed coke particles may be connsidered' sis-introducing fissures orrsmall channelsv throughout theblack ash pile;- By virtue-offthis;

air supplied to the furnacereadily and quickly er heat in the furnace-for the generation of steam:-

The'sulphur contained in-the'coke-will also-melt to add more sulphur to the smelt;

Preferably we employ-a weight-of cokebetweer-r 6% and 8% of the total weight of the dry solids supplied to the furnace. This weight is substantially the same as the weight of salt cake supplied. If it be considered that coke has an approximate heating value of about 14,500 B. t. u.s per lb. and that for each 3000 lbs. of dry solids contained in the black liquors and fed to the furnace, 200 lbs. of coke is supplied, there is added to the furnace approximately 970 B. t. u.s for every lb. of dry solids. Assuming that the dry solids have a heating value of 6100 B. t. u., then, with the addition of the coke, there is obtained in 1.06 lbs. of fuel supplied, a heating value of 7070 B. t. u., or a new substance having a heating value of 6700 B. t. u. approximately.

A furnace operated with the fuel value of 6100 B. t. u. is much more difiicult to operate than one with a fuel value of 6700 B. t. u., as the added 600 B. t. u. increases the amount of gases and raises the furnace temperatures, producing what may be termed a hotter furnace. Thus the loss of heat value in the black liquors of recent practice is compensated for and the ashing conditions of the fuel bed are such as to counteract the presence of the additional chemical employed. A smelt having a controlled weight of sodium sulphide is obtained, producing a white liquor of high sulphidity wherewith to obtain improved fiber and yield. The operating difiiculties are overcome.

By reason of the controllable feeding of the salt cake and the coke, it is possible to deliver to the mixing tank or other place in the system, a ratio of coke and salt cake such as will give desired results With respect to both the ashing and the sulphidity. It is also possible to change this ratio at will by an increase or decrease of the amount of salt cake and coke introduced; and to increase or decrease the total amount of salt cake and coke, in accordance with the rate at which it is desired to operate the furnace.

Thus the aforementioned operating difficulties are overcome, a high yield of chemical is obtained, and the needed control of sulphidity achieved.

The chemical withdrawn from the smelter furnace is, as hereinbefore pointed out, dissolved in a solution of weak liquor and the resultant mixture becomes known as green liquor. When the slaked lime is added thereto, the so-called white liquor results and is ready for reuse as the sol vent in the digester process. The green liquor is first led to a decanting tank (not shown), in which tank the liquors are settled to remove all suspended solids before they are sent to the causticizing tank or tanks in which the white liquor mixture is formed. Hence the coke. ash produced in the smelting operation is settled out and drawn off to the refuse piles with the other undesirable grit or dirt found in all green liquors.

cokee ash: withdrawn with; the smelt: stream has: aahighenrfusing': temperature athan the tem-- peraturea- Off the: smelha. which; latter; is around; 1600??" Therefore; when: the smelt and coke; ashes; strikes the: liquors; Off the 1 main; dissolving tank;. .they:are'ichilled, the-temperatures .prevail--- ingzimthe dissolvingztankrbeing irrthe neighbor hood of 1805"E.--. In1-view:-of the foregoing; it .will be seen that nozdifficultya-is encountered by reason of the asltcontent-offthe coke or other fuel used.

wee haveaindicated our preference of spraying the black liquor, into the furnace in space, and while we' have indicated preferred temperaturesand, pressuresat the nozzles, it is-to be understood that the invention-isnot limited:

thereto and that the concept of enriching the black liquor and the controlling of the sulphidity, will be applicable to other conditions and modes of firing.

We claim:

1. In the continuous smelting process of recovering chemical from the black liquor derived in pulp mills from digesting the cellulose with white liquor, in which the black liquor in concentrated and then substantially continuously sprayed into a boiler furnace to gravitate therein to be dried for combustion of the combustible constituents thereof and in which salt cake is substantially continuously introduced into the black liquor prior to its introduction into the furnace and in which the chemical is recovered as a smelt in the furnace and the smelt is treated to obtain white liquor for re-use in the digestion of further cellulose, the step of controlling the sulphidity of the White liquor obtained from the smelt by substantially continuously introducing coke in particle form into the black liquor prior to the introduction of such black liquor into the furnace and thereby also assisting in making up any deficiency of B. t. u. of the black liquor for self-sustaining operation.

2. In the continuous smelting process of recovering chemical from the black liquor derived in pulp mills from digesting the cellulose with white liquor, in which the black liquor is concentrated and then substantially continuously sprayed into a boiler furnace to gravitate therein to be dried and for combustion of the combustible constituents thereof and in which salt cake is substantially continuously introduced into the black liquor prior to its introduction into the furnace and in which the chemical is recovered as a smelt in the furnace and the smelt is treated to obtain white liquor for re-use in the digestion of further cellulose and in which soap is removed from the black liquor, the step of simultaneously controlling sulphidity of the white liquor so obtained and replacing the B. t. u.s lost by removal of the soap by substantially continuously introducing coke in particle form into the black liquor prior to its delivery to the furnace in an amount compensating for the reduction in B. t. u.s in the black liquor incident to the removal of the soap.

3. In the continuous smelting process of recovering chemical from the black liquor derived in pulp mills from digesting the cellulose with white liquor, in which the black liquor is concentrated and then substantially continuously sprayed into recovered as a smelt in the furnace and the smelt." is treated to obtain white liquor for re-use in the digestion of further cellulose, the step of controlling the sulphidity of white liquor obtained from the smelt by substantially continuously introducing coke in particle form into the black liquor at the point where the salt cake is introduced and mixed into the black liquor.

E. GAGLIARDI. CHARLES MELLOR. ALEXANDER L. HAMM.

REFERENCES CITED The following references are of record in the file of this patent:

Number 

1. IN THE CONTINUOUS SMELTING PROCESS OF RECOVERING CHEMICAL FROM THE BLACK LIQUOR DERIVED IN PULP MILLS FROM DIGESTING THE CELLULOSE WITH WHITE LIQUOR, IN WHICH THE BLACK LIQUOR IN CONCENTRATED AND THEN SUBSTANTIALLY CONTINUOUSLY SPRAYED INTO A BOILER FURNACE TO GRAVITATE THEREIN TO BE DRIED FOR COMBUSTION OF THE COMBUSTIBLE CONSTITUENTS THEREOF AND IN WHICH SALT CAKE IS SUBSTANTIALLY CONTINUOUSLY INTRODUCED INTO THE BLACK LIQUOR PRIOR TO ITS INTRODUCTION INTO THE FURNACE AND IN WHICH THE CHEMICAL IS RECOVERED AS A SMELT IN THE FURNACE AND THE SMELT IS TREATED TO OBTAIN WHITE LIQUOR FOR RE-USE IN THE DIGESTION OF FURTHER CELLULOSE, THE STEP OF CONTROLLING THE SULPHID- 